The described invention relates in general to ultrasonic welding systems and more specifically to a device and method for enhancing the performance of sonotrodes used in ultrasonic additive manufacturing applications.
Ultrasonic welding is an industrial process involving high-frequency ultrasonic acoustic vibrations that are locally applied to workpieces being held together under pressure to create a solid-state weld. This process has applications in the electrical/electronic, automotive, aerospace, appliance, and medical industries and is commonly used for plastics and especially for joining dissimilar materials. Ultrasonic welding of thermoplastics results in local melting of the plastic due to absorption of vibration energy. The vibrations are introduced across the joint to be welded. In metals, ultrasonic welding occurs due to high-pressure dispersion of surface oxides and local motion of the materials. Although there is heating, it is not enough to melt the base materials. Vibrations are introduced along the joint being welded.
Ultrasonic welding systems typically include the following components: (i) a press to apply pressure to the two parts to be assembled under pressure; (ii) a nest or anvil where the parts are placed for allowing high frequency vibration to be directed to the interfaces of the parts; (iii) an ultrasonic stack that includes a converter or piezoelectric transducer for converting the electrical signal into a mechanical vibration, an optional booster for modifying the amplitude of the vibration (it is also used in standard systems to clamp the stack in the press), and a sonotrode or horn for applying the mechanical vibration to the parts to be welded (note: all three components of the stack are specifically tuned to resonate at the same exact ultrasonic frequency which is typically 20, 30, 35 or 40 kHz); (iv) an electronic ultrasonic generator or power supply delivering a high power AC signal with frequency matching the resonance frequency of the stack; and (v) a controller for controlling the movement of the press and the delivery of the ultrasonic energy.
In an exemplary system, the power supply provides high-frequency electrical power to the piezoelectric-based transducer, creating a high-frequency mechanical vibration at the end of the transducer. This vibration is transmitted through the booster section, which may be designed to amplify the vibration, and is then transmitted to the sonotrode, which transmits the vibrations to the workpieces. The workpieces, usually two thin sheets of metal in a simple lap joint, are firmly clamped between the sonotrode and a rigid anvil by a static force. The top workpiece is gripped against the moving sonotrode by a knurled pattern on the sonotrode surface. Likewise, the bottom workpiece is gripped against the anvil by a knurled pattern on the anvil. The ultrasonic vibrations of the sonotrode, which are parallel to the workpiece surfaces, create the relative frictionlike motion between the interface of the workpieces, causing the deformation, shearing, and flattening of surface asperities. Welding system components, commonly referred to as the transmission line or “stack” are typically housed in an enclosure case that grips the welding assembly at critical locations (most commonly the anti-node) so as to not dampen the ultrasonic vibrations, and to provide a means of applying a force to and moving the assembly to bring the sonotrode into contact with the workpieces and apply the static force.
A number of parameters can affect the welding process, such as ultrasonic frequency, vibration amplitude, static force, power, energy, time, materials, part geometry, and tooling. With regard to tooling, which includes the sonotrode, welding tip, and the anvil, these components support the parts to be welded and transmit ultrasonic energy and static force. The welding tip is usually machined as an integral part of a solid sonotrode. The sonotrode is exposed to ultrasonic vibration and resonates in frequency as “contraction” and “expansion” x times per second, with x being the frequency. The amplitude is typically a few micrometers (about 13 μm to 130 μm). The shape of the sonotrode (round, square, with teeth, profiled, etc), depends on the quantity of vibratory energy and a physical constraint for a specific application. Sonotrodes are made of titanium, aluminum or steel. For an ultrasonic welding application, the sonotrode provides energy directly to the welding contact area, with little diffraction. This is particularly helpful when vibrations propagation could damage surrounding components.
Ultrasonic additive manufacturing (UAM) is an additive manufacturing technique based on the ultrasonic welding of metal foils and computer numerically controlled (CNC) contour milling. UAM can also be characterized as a solid-state metal deposition process that allows build-up or net-shape fabrication of metal components. High-frequency (typically 20,000 hertz) ultrasonic vibrations are locally applied to metal foil materials, held together under pressure, to create a solid-state weld. CNC contour milling is then used to create the required shape for the given layer. This process is then repeated until a solid component has been created or a feature is repaired or added to a component. UAM can join dissimilar metal materials of different thicknesses and allow for the embedment of fiber materials at relatively low temperature, (typically <50% of the metal matrix melting temperature) and pressure into solid metal matrices.
Current UAM technology utilizes titanium based tools which tend to wear rapidly, often resulting in a loss of displacement of the target media due to insufficient interaction of worn texture profiles during the ultrasonic welding process. Deflection of the sonotrode and loss of displacement under various forces can significantly affect the bond quality of build-ups of metal components during the UAM process. Incorporation of advanced tool steels into modified sonotrode designs would permit higher, more uniform stress distribution in the system, thereby allowing higher static forces to be applied to advanced materials while retaining critical surface texturing over extended periods of time. Therefore, there is a need for a sonotrode design that assists the UAM welding process by generating higher static forces required for transmitting increased levels of ultrasonic energy useful for producing components that include Ni, Ti, or high speed steel (HSS).